Cleaning of hollow fibre membranes

ABSTRACT

PCT No. PCT/AU95/00587 Sec. 371 Date Aug. 8, 1997 Sec. 102(e) Date Aug. 8, 1997 PCT Filed Sep. 8, 1995 PCT Pub. No. WO96/07470 PCT Pub. Date Mar. 14, 1996A method and apparatus for recovering fine solids from a liquid feed suspension is disclosed. The apparatus has an operating cycle including a concentration part of the cycle in which solids present in the feed suspension are concentrated and a backwash part of the cycle in which supply of feed suspension to the concentrator is interrupted, the concentrator comprises a shell (11), and a plurality of elastic, hollow, microporous, polymer fibers (12) being fixed at their ends within the shell (11). Pressurized feed suspension is supplied to the outside of the fibers during said concentration part of the cycle and the filtrate may be withdrawn from the fiber lumens during the operating cycle. During the backwash cycle the concentration part of the cycle is terminated by ceasing supply of feed to said exterior surface of the fibers (12). The shell (11) is then sealed and the remaining filtrate removed from the lumens. A source of fluid under pressure is then applied to said lumens before, at the same time as, or just after opening the shell (11) to atmosphere, to cause explosive decompression through the walls of the fibers (12) whereby the fluid under pressure passes through said walls. The pressure level in said lumens is maintained at a predetermined value for a sufficient time following said decompression to cause substantial portions of solids lodged within and/or on the fiber walls to be dislodged. The dislodged contaminant matter is then washed away by the application of a flow of liquid over the surface of the fiber walls; and the concentration part of the operating cycle is recommended by introducing the supply of feed suspension to said exterior surface of said fibers (12).

TECHNICAL FIELD

The present invention relates to concentration of solids in a suspensionusing a hollow fibre membrane and, in particular forms, to methods andapparatus for periodically cleaning by backwashing the hollow fibremembranes.

BACKGROUND ART

Prior art methods of concentrating solids in a liquid suspension aredescribed in Australian patent specifications 576,424 and 582,968. Thetext and drawings of these specifications are incorporated herein bycross-reference. In that prior art, concentration is effected by afilter element that comprises a bundle of hollow, porous, polymericfibres in a closed cartridge or shell. Polyurethane potting compound isused to hold the respective ends of the fibres in place within thecartridge without blocking the fibre lumens and to close off each end ofthe cartridge.

The transmembrane pressure differential necessary to effectconcentration of the solids in the prior art is achieved by pressurisingthe feedstock which necessitates the use of pumps, other ancillaryequipment and, of course, a closed filter cartridge.

Backwashing of such prior art concentrators involves increasing thepressure on both sides of the hollow fibres within the closed shell to arelatively high value before suddenly releasing that pressure on theshell side of the fibre walls to effect a sudden pressure differentialacross the walls which causes a backwash action.

DISCLOSURE OF INVENTION

It is an object of this invention to provide an improved method of usinga reverse-flow mode to dislodge solids retained by filter elements toensure rapid removal of those retained solids and in which theseparation and dislodgement modes may be repeated for prolonged periodsof time.

The present invention, in at least some embodiments, provides a methodof backwashing a hollow fibre filter which retains some of the featuresof the prior art, but optimizes a number of these features to provideimproved performance.

Accordingly, in one broad form of the invention, there is provided amethod of backwashing a plurality of hollow fibres having microporouswalls which have been subjected to a filtration operation wherein feedcontaining contaminant matter is applied to the exterior surface of saidhollow fibres and filtrate withdrawn from the ends of the lumens of thefibres, the fibres being contained within a shell or housing, saidmethod comprising:

(a) terminating the filtration operation by ceasing supply of feed tosaid exterior surface of said fibres,

(b) sealing the shell and substantially removing remaining filtrate fromsaid lumens,

(c) applying a source of fluid under pressure to said lumens before, atthe same time as, or just after opening the shell to atmosphere, tocause explosive decompression through the walls of the fibres wherebysaid fluid under pressure passes through said walls;

(d) maintaining the pressure level in said lumens at a predeterminedvalue for a sufficient time following said decompression to causesubstantial portions of contaminant matter lodged within and/or on saidfibre walls to be dislodged;

(e) washing dislodged contaminant matter away by the application of aflow of liquid over the external surface of said fibre walls; and

(f) recommencing the filtration operation by introducing said supply offeed to said exterior surface of said fibres.

For preference, during the backwash phase, the time lapse between thestart of an increase in negative transmembrane pressure (TMP) and suchTMP reaching a maximum value corresponding to the explosivedecompression, is in the range of about 0.05 seconds to about 5 seconds.

Preferably, the fibres are rewetted prior to recommencing the filtrationoperation.

Preferably, feed liquid is pumped into the shell side of the filterwhile fluid pressure is still being applied to said lumens. This resultsin liquid/fluid turbulence or fronting around the membrane pores causingfurther improved dislodgement of retained solids. The fluid pressureduring this phase preferably should exceed the shell side pressure byabout 10 kPa to about 800 kPa.

Preferably, the steps of the method are carried out as a continuousprocess utilizing repetitive cycles of solids retention and backwash.

As an alternative preferred form, step (b) is effected by allowing saidremaining filtrate to drain out of said lumens of its own volition.

According to a further broad aspect, the present invention provides aconcentrator for recovering fine solids from a liquid feed suspensionand having an operating cycle including a concentration part of thecycle in which solids present in the feed suspension are concentratedand a backwash part of the cycle in which supply of feed suspension tothe concentrator is interrupted, said concentrator comprising:

(i) a shell;

(ii) a plurality of elastic, hollow, microporous, polymer fibres beingfixed at their ends within the shell;

(iii) means for supplying pressurized feed suspension to the outside ofthe fibres during said concentration part of the cycle;

(iv) means for withdrawing filtrate from the fibre lumens during saidoperating cycle;

(v) means for terminating the concentration part of the cycle by ceasingsupply of feed to said exterior surface of said fibres;

(vi) means for sealing and opening the shell to atmosphere;

(vii) means for substantially removing remaining filtrate from saidlumens;

(viii) means for applying a source of fluid under pressure to saidlumens before, at the same time as, or just after opening the shell toatmosphere, to cause explosive decompression through the walls of thefibres whereby said fluid under pressure passes through said walls;

(ix) means for maintaining the pressure level in said lumens at apredetermined value for a sufficient time following said decompressionto cause substantial portions of solids lodged within and/or on saidfibre walls to be dislodged;

(x) means for washing dislodged contaminant matter away by theapplication of a flow of liquid over the external surface of said fibrewalls; and

(xi) means for recommencing the concentration part of the operatingcycle by introducing said supply of feed suspension to said exteriorsurface of said fibres.

Preferably, the concentrator includes means for rewetting the fibresbefore recommencing concentration.

Where fluid pressure is applied to remove the filtrate from the lumens,this pressure is typically in the range of about 10 to about 600 kPa.The fluid pressure applied to the lumens prior to the decompression istypically in the range of about 100 to about 1200 kPa.

The penetration of gas into the pores of a membrane is resisted by thesurface tension forces of the contained wall-wetting liquid according towell known theory. Indeed, surface tension is conveniently measured bythe breakthrough pressure needed to force a bubble out of a submergedorifice. For common systems (such as oil in hydrophobic pores or waterin hydrophilic pores) the breakthrough pressures are much higher thanthe usual operating pressures of the filter.

Prior art hollow-fibre type ultrafilters are usually fed from the insideof the fibres for many well known reasons. However, according to thepresent invention, feedstock is applied to the outside of the fibres andgas is introduced into the lumen of the fibre as the back-wash medium.In some cases, the lumen pressure swells a suitably designed fibre sothat the pores are enlarged whereby the particles are freed and sweptaway in the expansion of the back-wash gas.

In some cases, especially where very fine-pored interstitial material isdeposited in relatively coarse-pored base fibre, it is advantageous toback-wash first with a small amount of permeate already in the membranelumen and follow with the high pressure gas back-wash. In this way, thesmall amount of permeate adequately washes out fine blocking materialfrom within the interstices, and the overall cleaning is completed bythe higher pressure gas swelling the base pores and erupting aroundelastic openings. The pores must close again rapidly to reseal the holesand the base material must not crack by work hardening and must remainwithin its modified elastic limit.

Preferably, the fibres are made from thermoplastic polymers such as:

poly(propylene), poly(4-methylylpent-1-ene), co-polymers ofpolypropylene, poly(vinylidenedifluoride), poly(sulphones),poly(phenylene sulphides), poly(phenylene oxides), phenoxy resins,polyethylene, poly(tetrafluoroethylene) andpoly(chlorotrifluoroethylene).

The use of gas as a back-wash medium enables the removal of foulingspecies by explosive decompression of the gas through the membranestructure for the minor part and at the outer membrane surface for themajor part. Thus, the gaseous back-wash step is carried out at apressure which is sufficient to overcome the effect of the surfacetension of the continuous phase of the feedstock within the pores of themembrane.

Hitherto, it was felt the gas backwashing phase should be limited tobelow 5 seconds to avoid drying out of the fibres and thus difficulty inrecommencing filtration due to gas bubble retention in the fibre pores.The introduction of improved rewetting techniques has overcome thisproblem and it has been discovered that extending the gas backwash phasebeyond 5 seconds has significant advantages. Time periods of up to 60seconds have been found to be effective. A longer backwash providesimproved removal of trapped solids. Also, where liquid is reintroducedto the shell prior to completion of the gas backwash, it has enabled theoverlap where gas and liquid are both present to be extended. An overlaptime of about 1 to about 30 seconds is preferable. This is desirable inlarge arrays where it may take considerable time, with normal pumppressures, to refill the shells with liquid. The extended time periodenables normal pumps to be used to achieve the above overlap while italso avoids maldistribution of pressure within large filter arrays byallowing relatively slow refilling of the filter shells.

In another form of the invention, the high pressure fluid application tothe lumens may be pulsed to provide a number of explosive decompressionswithin the backwashing phase. These individual pulses are preferablybetween about 0.1 seconds and about 5 seconds in duration. This providesan advantage of reducing gas consumption in the backwash phase. Thepulsing may be achieved by sealing and opening the shell at appropriatetime intervals sufficient to allow pressure within the lumens to buildup to a required level. Alternatively, the pressure supply may be pulsedto achieve the same effect. In a further embodiment, the pressure may bevaried between a high and low level without actual total shut off ofpressure.

BRIEF DESCRIPTION OF DRAWINGS

Preferred embodiments of the present invention will now be described, byway of example only, with reference to the following examples andaccompanying drawings, in which:

FIG. 1 shows a schematic representation of a hollow fibre cross-flowconcentrator to which the present invention is applicable in anoperating mode;

FIG. 2 shows the concentrator of FIG. 1 in backwash mode;

FIG. 3 shows a graph of transmembrane pressure (TMP) versus time for astandard backwash;

FIG. 4 shows a similar graph to FIG. 3 using a backwash according to oneaspect of the invention;

FIG. 5 shows a normalized flow/TMP versus time graph for a standardbackwash;

FIG. 6 shows a normalized flow/TMP versus time graph for the same typeof machine as FIG. 5 but using the backwash according to one aspect ofthe invention;

FIG. 7 shows a graph of TMP versus time for a backwash where feed liquidis pumped into the filter while the gas backwash is still applied;

FIG. 8 shows a normalized flow/TMP versus time graph for a standardbackwash;

FIG. 9 shows a normalized flow/TMP versus time graph for the same typeof machine as FIG. 8 but introducing feed liquid during the backwashcycle;

FIG. 10 shows a normalized flow/TMP versus time graph for a standardbackwash at a further installation; and

FIG. 11 shows a normalized flow/TMP versus time graph for the same typeof machine as FIG. 10 but introducing feed liquid during the backwashcycle.

MODES FOR CARRYING OUT THE INVENTION

The hollow fibre cross-flow concentrator 10 shown in FIGS. 1 and 2includes a cartridge shell 11 within which is positioned a bundle ofhollow, porous, polymeric fibres 12. In this instance, each fibre ismade of polypropylene, has an average pore size of 0.2 μm, an internallumen diameter in the range 250 μm to 310 μm and a fibre diameter in therange 500 μm to 650 μm. There may be between 2,800 to 30,000 hollowfibres in the bundle 12 but this number as well as the individual fibredimensions may be varied according to operational requirements.

Polyurethane potting compound 13,14 holds the ends of the fibres 12 inplace without blocking their lumens and closes off each end of the shell11. The liquid feed suspension to be concentrated is pumped into theshell 11 through feed suspension inlet 15 and passes over the externalwalls of the hollow fibres 12. Some of the feed suspension passesthrough the walls of the fibres 12 into the lumens of the fibres to bedrawn off through the lumen outlet port 16 as clarified liquid.

The remaining feed suspension and some of the rejected species flowsbetween the fibres 12 and leaves the shell 11 through outlet 17. Theremainder of the rejected species is held onto or within the fibres oris otherwise retained within the shell. Lumen inlet port 18 remainsclosed during the operating mode of the concentrator shown in FIG. 1.

In order to remove the retained species, lumen outlet port 16 is closedso that the flow of clarified liquid is stopped. The clarified liquid isthen removed from the lumens by natural drainage or by introducing apressurized gas through lumen inlet port 18 to force the liquid from thelumens. Upon completion of the removal of the filtrate liquid, highpressure compressed gas is introduced through inlet 18 and the lumens ofthe fibres 12. The liquid-filled shell is sealed and gas cannotpenetrate the porous walls even though the gas pressure is now raisedwell above the normal bubble point of the fibre walls because the liquidwithin the shell is relatively incompressible. A reservoir of highpressure gas is thus accumulated in the fibre lumens.

The shell outlet 17 is then opened which allows gas to penetrate thepores along the whole length of each fibre. This results in a explosivedecompression of the pressurized gas through the walls of the fibresresulting in the retained solids in the fibre walls being dislodged fromthe fibres into the feed side of the filter. The initial breakthrough ofgas through the fibre wall results in a tendency for pressure to drop inthe lumens. It is desirable if this pressure can be maintained for ashort period following decompression to cause increased flow through thefibre wall and greater removal of retained solids. This is preferablyachieved by providing a large diameter pressure feed to the lumensand/or a higher pressure to compensate for pressure drop. In some cases,it is desirable to admit gas through both lumen ports 16 and 18 aftercarrying out the above described pressurised, trapped gas operation.

In alternate embodiments, the shell is opened just before or at the sametime as the pressurized gas is applied to the lumens.

Referring to the accompanying graphs, a number of examples will now bedescribed to illustrate the improved performances provided byembodiments of the invention.

EXAMPLE 1

An M10C (250 μm lumen) filter unit was run using a larger airline toprovide an increased and prolonged pressure to the lumens following theexplosive decompression phase. A 2.5 cm airline was used instead of astandard 10 mm airline. There was no pressurize stage used during thisimproved backwash and the negative transmembrane pressure (TMP) obtainedon the filter unit was 620 kPa compared with 380 kPa for a standardbackwash. The air consumption was higher than that for a standardbackwash. The pressure profiles of the two different backwashes areshown in FIGS. 3 and 4.

During the standard backwash shown in FIG. 3, it can be seen that a timeof 0.65 seconds elapses between the start of the explosive decompressionphase and the point at which maximum TMP is obtained. Analysis of thesimilar section of the improved backwash shows the time to reach maximumnegative TMP was only 0.15 seconds. The reaching of maximum TMPcorresponds with the air breaking through the walls of the fibre andexpelling the fluid within the wall pores. The period between theopening of the shell and the breakthrough is a liquid backwash phase asthe liquid within the pores is being moved outwardly from the lumentoward the shell side. When the air breaks through the fibre wall theliquid backwash phase is completed. Preferably this period is within therange 0.05 seconds to 5 seconds.

The results of consecutive runs on the test filter unit comparing thestandard and the improved backwash (termed a "mega" backwash herein)procedures are shown in attached TABLE 1.

As can be seen from TABLE 1 and the performance graphs (FIGS. 5 and 6),the TMP rise is significantly reduced when the "mega" backwash is used.The TMP rise per day for the `mega` backwash was approximately onequarter of the TMP rise seen with the standard backwash. This resultmeans that machines could be run for longer between cleaning cycles, orthe machines could give a higher throughput for the same cleaninginterval.

EXAMPLE 2

This example relates to the procedure where feed liquid is reintroducedto shell while the gas backwash is still proceeding. A trial was carriedout on surface water to compare a standard backwash with a backwashstage using pressurized gas plus feed liquid. This stage is typicallyreferred to as an "air on pump on" stage (AOPO stage).

Two identical 1M10C filter units were set up to run side by side onriver water. One machine used a standard pressurize backwash cycle,whilst the other incorporated an extra stage. The extra stage consistedof switching on the feed pump whilst still applying high pressure airthrough the hollow fibre walls. The resultant two phase flow across thefibre bundle appeared to be very effective in removing fouling from themembrane module.

The two filter units were running at a constant flow of 200 L/hr/em,using a pump with a variable speed drive to keep the set flow. The areaunit em is related to the surface area of an original Memtec filtermodule. FIGS. 8 and 10 illustrate the results of two consecutive runs ofa filter unit and show that the TMP, when the standard backwash wasused, rose to 400 kPa within 4 days of operation. At this point the unitcould no longer maintain the set flow of 200 L/hr/em. FIGS. 9 and 11show that when the AOPO stage was used the TMP remained below 150 kPafor 7 days. The result of this is that the filter units could maintain ahigher flow rate for a longer period of time when the AOPO stage is usedin the backwash. This is important to filter unit efficiency as theunits require chemical cleaning when the TMP reaches a predeterminedvalue.

Typically during the backwash the decompression stage consists of thelumens being pressurized to 600 kPa, then the shell side valves beingreleased whilst still supplying air to the lumens (for typically 1 to 3seconds on most applications). The AOPO stage would extend the amount oftime air is resupplied to the fibre lumens by typically an extra 1 to 30seconds on M10 units.

It will be appreciated that further embodiments and exemplifications ofthe invention are possible without departing from the spirit or scope ofthe invention described.

                  TABLE 1                                                         ______________________________________                                        M10C (250 μm lumens) comparison of `mega` and                                standard backwashes                                                           BACKWASH                                                                      TYPE STANDARD MEGA                                                          ______________________________________                                        Module Type                                                                              PP M10C (20,000 fibres)                                                                      PP M10C (20,000 fibres)                               Feed Type River water River water                                             Feed Turbidity 8   7                                                          (NTU)                                                                         Feed Temperature 9.3 9.3                                                      (° C.)                                                                 TMP (kPa)  103  92                                                            TMP Range (kPa) 82 to 108 86 to 91                                            Instantaneous Flow* 2084 2223                                                 (L/hr/module)                                                                 Instantaneous Flow* 2855 3045                                                 (L/hr/module) at                                                              20° C.                                                               ______________________________________                                         *Instantaneous flow is the average of instantaneous flowrates measured.  

We claim:
 1. A method of backwashing a plurality of hollow fibres havingmicroporous walls which have been subjected to a filtration operationwherein feed containing contaminant matter is applied to the exteriorsurface of said hollow fibres and filtrate withdrawn from the ends ofthe lumens of the fibres, the fibres being contained within a shell orhousing, said method comprising:(a) terminating the filtration operationby ceasing supply of feed to said exterior surface of said fibres, (b)sealing the shell and substantially removing remaining filtrate fromsaid lumens, (c) applying a source of fluid under pressure to saidlumens before, at the same time as, or just after opening the shell toatmosphere, to cause explosive decompression through the walls of thefibres whereby said fluid under pressure passes through said walls; (d)maintaining the pressure level in said lumens at a predetermined valuefor a sufficient time following said decompression to cause substantialportions of contaminant matter lodged within and/or on said fibre wallsto be dislodged; (e) washing dislodged contaminant matter away by theapplication of a flow of feed liquid over the external surface of saidfibre walls; and (f) recommencing the filtration operation byreintroducing said supply of feed to said exterior surface of saidfibres while fluid pressure is still being applied to said lumens.
 2. Amethod of backwash according to claim 1 wherein the fibres are rewettedprior to reconnecting the filtration operation.
 3. A method ofbackwashing according to claim 1, wherein during step (c), the lapsetime between the start of application of fluid under pressure to theexplosive decompression is in the range of about 0.05 seconds to about 5seconds.
 4. A method of backwashing according to claim 3 wherein thefibres are rewetted prior to recommencing the filtration operation.
 5. Amethod of backwashing according to claim 1 wherein feed liquid isreintroduced for a period of between about 1 to about 30 seconds.
 6. Amethod according to claim 1 wherein said fluid pressure exceeds shellside pressure by about 10 kPa to about 800 kPa.
 7. A method ofbackwashing according to claim 1 wherein the pressure applied to saidlumens prior to said explosive decompression is between about 100 kPa toabout 1200 kPa.
 8. A method according to claim 1 wherein during step (b)the filtrate is allowed to drain out of said lumens of its own volition.9. A method according to claim 1 wherein the steps of the method arecurried out as a continuous process utilizing repetitive cycles ofsolids retention and backwash.